CO2 Reduction by Photocatalysis on TiO2

Introduction

Before a new popular energy source of clean style are available, fossil fuels, including combustible gas, crude oil, and coal currently are still the main sources for the production of useful chemicals and energy until now. Statistics shows that most of the produced energy was generated from fossil fuels since 2011, which will domain the energy source for a long time until around 2030 by a prediction referenced to a BP’s energy outlook. The energy demand in the world is forecasted with a growth by 36% from 2011 to 2030 (Kondratenko, Mul, Baltrusaitis, Larrazábal, & Pérez-Ramírez, 2013). As CO₂ is a final product of the fossil combustion process, the back conversion and utilization of CO₂ are essential areas of conservation of resources and the development of a sustainable society (Lewis & Nocera, 2006; Schlögl, 2015). Therefore, decreasing CO₂ amount in the air is of significance to address problems relating to the ‘greenhouse effect’. On the other hand, it will be interesting when using CO₂ as the chemical source in conversion to solar fuels (Gust, Moore, & Moore, 2009). Furthermore, solar fuels generated from CO₂ have advantages in storage and manipulation in comparison with hydrogen technology (Huber, Iborra, & Corma, 2006; K.-F. Li, An, Park, Khraisheh, & Tang, 2014).

However, even great progress has been achieved on photocatalysis technology dealing how to obtain chemical energy from photo energy, conversion of CO₂ is currently still a great challenge since CO₂ has a very stable chemical bonding which demands significant energy breaking the bonding on special catalysts. Numerous researchers have realized CO₂ can be converted into various carbonates, carboxylates, and carbamates by electrochemical or photochemical methods (Albero & García, 2016; Roy, Varghese, Paulose, & Grimes, 2010). To efficiently convert CO₂ into ideal fuel products, suitable activation mechanisms and reaction conditions must be found. Until now, chemical reduction of CO₂ requires a substantial energy input which limits the further development of CO₂ reduction technology.

Recently, photocatalytic reduction of CO₂ has attracted worldwide attention since its convenience, easy for controlling, low cost, and specified selectivity of organic production (Balzani, Credi, & Venturi, 2008; Morris, Meyer, & Fujita, 2009). This technology may ameliorate atmospheric CO₂ levels meanwhile provide useful chemical products such as carbon monoxide (CO), methane (CH₄), methanol (CH₃OH), formaldehyde (HCHO), and formic acid (HCOOH) (D. Liu et al., 2012; L. J. Liu, Zhao, Andino, & Li; Roy et al., 2010). In addition, if combined with the synthesis by solar energy, the light-induced reaction of CO₂ and H₂O to prepare hydrocarbons is a green technology for the conversion of solar energy to chemical energy. And the overall footprint of CO₂ is considered to be neutral if these hydrocarbons are generated from CO₂ as feedstock (Gust et al., 2009). Although many semiconductors have been investigated as photo catalysts for CO₂ reduction, TiO₂ has been proved to be the most suitable photocatalyst with the highest photocatalysis efficiency due to its stability and its relatively favorable bandgap energy. Great efforts have been made to increase the photocatalytic performance of TiO₂ for CO₂ conversion by various strategies.